Inhibition of granulocyte-macrophage colony-stimulating factor receptor function by a splice variant of the common beta-receptor subunit

The Truncated Splice Variant of the Granulocyte-Macrophage-Colony-Stimulating Factor Receptor β- Chain in Peripheral Blood Serves as Severity Biomarker of Respiratory Failure in Newborns

BACKGROUND

The granulocyte-macrophage-colony-stimulating factor (GM-CSF) plays an important role in surfactant homeostasis. βC is a subunit of the GM-CSF receptor (GM-CSF-R), and its activation mediates surfactant catabolism in the lung. βIT is a physiological, truncated isoform of βC and is known to act as physiological inhibitor of βC.

OBJECTIVE

The aim of this study was to determine the ratio of βIT and βC in the peripheral blood of newborns and its association with the degree of respiratory failure at birth.

METHODS

We conducted a prospective cohort study in newborns with various degrees of respiratory impairment at birth. Respiratory status was assessed by a score ranging from no respiratory impairment (0) to invasive respiratory support (3). βIT and βC expression were determined in peripheral blood cells by real-time PCR. βIT expression, defined as the ratio of βIT and βC, was correlated with the respiratory score.

RESULTS

βIT expression was found in all 59 recruited newborns with a trend toward higher βIT in respiratory ill (score 2, 3) newborns than respiratory healthy newborns ([score 0, 1]; p = 0.066). Seriously ill newborns (score 3) had significantly higher βIT than healthy newborns ([score 0], p = 0.010). Healthy preterm infants had significantly higher βIT expression than healthy term infants (p = 0.019).

CONCLUSIONS

βIT is expressed in newborns with higher expression in respiratory ill than respiratory healthy newborns. We hypothesize that βIT may have a protective effect in postnatal pulmonary adaptation acting as a physiological inhibitor of βC and, therefore, maintaining surfactant in respiratory ill newborns.

Cytokine receptor splice variants in hematologic diseases

Cytokine and cytokine receptors are important regulators of hematopoiesis. Hematopoietic stem cells (HSCs) and progenitors differentiate into the myeloid or lymphoid lineage in response to specific cytokines. Cell-type specific receptors are expressed on committed progenitors that bind to other late-acting cytokines that are involved in terminal differentiation of hematopoietic cells. In normal hematopoiesis, these receptors undergo alternative splicing and are developmentally regulated. Splicing changes can significantly affect the structure and function of the receptors resulting in alterations of either the extracellular ligand binding domain or the cytoplasmic signaling domain responsible for cellular growth and differentiation. Most alternatively spliced isoforms generally lose the ability to promote differentiation. Evidently, overexpression of naturally occurring cytokine receptor alternate isoforms are observed in multiple myeloid diseases such as myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), and polycythemia vera (PV). The purpose of this review is to introduce the various isoforms of key cytokine receptors that play a crucial role in myeloid development and their potential role in myeloid diseases.

Ott1 (Rbm15) regulates thrombopoietin response in hematopoietic stem cells through alternative splicing of c-Mpl

Thrombopoietin (Thpo) signaling through the c-Mpl receptor promotes either quiescence or proliferation of hematopoietic stem cells (HSCs) in a concentration-dependent manner; however, in vivo Thpo serum levels are responsive to platelet mass rather than HSC demands, suggesting additional regulation exists. Ott1 (Rbm15), a spliceosomal component originally identified as a fusion partner in t(1;22)-associated acute megakaryocytic leukemia, is also essential for maintaining HSC quiescence under stress. Ott1 controls the alternative splicing of a dominant negative isoform, Mpl-TR, capable of inhibiting HSC engraftment and attenuating Thpo signaling. Ott1, which associates with Hdac3 and the histone methyltransferase, Setd1b, binds to both c-Mpl RNA and chromatin and regulates H4 acetylation and H3K4me3 marks. Histone deacetylase or histone methyltransferase inhibition also increases Mpl-TR levels, suggesting that Ott1 uses an underlying epigenetic mechanism to control alternative splicing of c-Mpl. Manipulation of Ott1-dependent alternative splicing may therefore provide a novel pharmacologic avenue for regulating HSC quiescence and proliferation in response to Thpo.

Pronounced thrombocytosis in transgenic mice expressing reduced levels of Mpl in platelets and terminally differentiated megakaryocytes

We generated mice expressing a full-length Mpl transgene under the control of a 2-kb Mpl promoter in an Mpl−/− background, effectively obtaining mice that express full-length Mpl in the absence of other Mpl isoforms. These mice developed thrombocytosis with platelet levels approximately 5-fold higher than wild-type controls and markedly increased megakaryocyte numbers. The reintroduction of one wild-type Mpl allele restored normal platelet counts. We excluded the deletion of Mpl-tr, a dominant-negative isoform, as the underlying molecular cause for thrombocytosis. Instead, we found that transgene expression driven by the 2-kb Mpl promoter fragment was decreased during late megakaryocyte maturation, resulting in strongly diminished Mpl protein expression in platelets. Because platelets exert a negative feedback on thrombopoiesis by binding and consuming Tpo in the circulation through Mpl, we propose that the severe reduction of Mpl protein in platelets in Mpl-transgenic Mpl−/− mice shifts the equilibrium of this feedback loop, resulting in markedly elevated levels of megakaryocytes and platelets at steady state. Although the mechanism causing decreased expression of Mpl protein in platelets from patients with myeloproliferative disorders differs from this transgenic model, our results suggest that lowering Mpl protein in platelets could contribute to raising the platelet count.

Expression of interleukin-10 splicing variants is a positive prognostic feature in relapsed childhood acute lymphoblastic leukemia

PURPOSE

Biologic features of hematologic malignancies have prognostic implications and are essential elements in the design of current therapeutic trials. This study aimed to determine the expression of a splicing-derived variant of interleukin (IL) -10 in leukemic cells and its clinical relevance in children with acute lymphoblastic leukemia (ALL) at first relapse.

PATIENTS AND METHODS

Between January 1997 and December 2001, bone marrow (BM) samples were collected from 98 children with first relapse of ALL at diagnosis. These patients were enrolled in the relapse trial ALL-REZ BFM (ALL-Relapse Berlin-Frankfurt-Munster) 95 and 96. The detection of IL-10 isoforms in leukemic cells of BM samples were performed by conventional reverse transcriptase polymerase chain reaction and by immunoblotting.

RESULTS

IL-10 was detected in 93.9% BM samples. In addition to expressing full-length IL-10, a new splicing-derived IL-10 variant (termed IL-10delta3) that lacked the entire exon 3 was identified in leukemic cells. The IL-10delta3 variant was found in 80.4% of BM samples. Most importantly, expression of IL-10delta3 was associated with a significantly better response to chemotherapy (P = .001) and probability of event-free survival (P = .01) at 5 years.

CONCLUSION

These results indicate that splicing-derived IL-10 isoforms may modulate IL-10-mediated biologic effects and therapeutic efficacy in lymphatic disease, and expression of IL-10delta3 is a positive prognostic feature in relapsed childhood ALL.

Hematopoietic cell transplantation for congenital bone marrow failure

PURPOSE OF REVIEW

Congenital bone marrow failure is rare and multifactorial. This review focuses on the outcome after allogeneic hematopoietic cell transplantation for the treatment of these disorders, with particular emphasis on recent discoveries and the challenges.

RECENT FINDINGS

In the treatment of congenital bone marrow failure disorders, the goals are to eliminate or reduce early and late toxicities and the risk of graft-versus-host disease. Novel nonmyeloablative fludarabine-based preparative regimens have demonstrated low risks of toxicity and acceptable engraftment rates for several congenital bone marrow failure disorders. Although there seems to be less early toxicity, longer follow-up is needed to determine late effects, especially the development of malignancy. T cell depletion of the bone marrow or peripheral blood, or the use of umbilical cord blood, has decreased the risk of graft-versus-host disease. Together, reduced toxicity and low rates of graft-versus-host disease have at least minimized the morbidity early after transplantation, with promising early survival.

SUMMARY

With marked improvement in rates of survival after allogeneic hematopoietic cell transplantation for selected congenital bone marrow failure disorders, emphasis is now being placed on improving quality of life and reducing late effects. Multicenter collaborative trials will determine the best treatment for these rare disorders.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

A truncated isoform of c-Mpl with an essential C-terminal peptide targets the full-length receptor for degradation

Thrombopoietin and its cognate receptor c-Mpl are the primary regulators of megakaryopoiesis and platelet production. They also play an important role in the maintenance of hematopoietic stem cells. Here, we have analyzed the function of a truncated Mpl receptor isoform (Mpl-tr), which results from alternative splicing. The mpl-tr variant is the only alternate mpl isoform conserved between mouse and humans, suggesting a relevant function in regulating Mpl signaling. Despite the presence of a signal peptide and the lack of a transmembrane domain, Mpl-tr is retained intracellularly. Our results provide evidence that Mpl-tr exerts a dominant-negative effect on thrombopoietin-dependent cell proliferation and survival. We demonstrate that this inhibitory effect is due to down-regulation of the full-length Mpl protein. The C terminus of Mpl-tr, consisting of 30 amino acids of unique sequence, is essential for the suppression of thrombopoietin-dependent proliferation and Mpl protein down-regulation. Cathepsin inhibitor-1 (CATI-1), an inhibitor of cathepsin-like cysteine proteases, counteracts the effect of Mpl-tr on Mpl protein expression, suggesting that Mpl-tr targets Mpl for lysosomal degradation. Together, these data suggest a new paradigm for the regulation of cytokine receptor expression and function through a proteolytic process directed by a truncated isoform of the same receptor.

Inhibition of GM-CSF receptor function by stable RNA interference in a NOD/SCID mouse hematopoietic stem cell transplantation model

RNA interference (RNAi) describes a highly conserved mechanism of sequence-specific posttranscriptional gene silencing triggered by double-stranded RNA (dsRNA). Whereas RNAi is applied to study gene function in different organisms and in variant cell types, little is known about RNAi in human hematopoietic stem and progenitor cells and their myeloid progeny. To address this issue, short hairpin RNAs (shRNA) were designed to target the common beta-chain of the human receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 (betaGMR). These receptors regulate proliferation, survival, differentiation, and functional activity of hematopoietic cells. In addition to markedly inhibiting mRNA and protein expression, anti-beta-GMR shRNAs were also found to inhibit receptor function in a cell culture model. Furthermore, lentiviral gene transfer of shRNA expression cassettes into primary normal CD34+ cells selectively inhibited colony formation of transduced progenitors when stimulated with GM-CSF/IL-3 but not when stimulated with cytokines that do not signal via beta-GMR. Finally, anti-beta-GMR shRNAs had no detectable effect on engraftment or lineage composition of lentivirally transduced human CD34+ cells transplanted into NOD/SCID mice. However, the growth defect of transduced colony-forming cells under stimulation with GM-CSF/IL-3 remains unchanged in bone marrow cells harvested from individual NOD/SCID mice 6 weeks after transplantation. These data indicate that lentiviral gene transfer of shRNA expression cassettes may be used to induce long-term RNAi in human hematopoietic stem and progenitor cells for functional genetics and potential therapeutic intervention.

Umbilical-cord blood transplantation for the treatment of cancer

Haematopoietic stem-cell transplantation is used to treat many haematological cancers, but is limited by the lack of suitable bone-marrow donors, the risk of graft-versus-host disease (GVHD) and slow immune reconstitution. Umbilical-cord blood is an alternative source of haematopoietic stem cells that has recently been tested in both child and adult cancer patients. These studies have identified several advantages to umbilical-cord cell transplantation, including a lower incidence of GVHD. Umbilical-cord blood is therefore a promising alternative to bone-marrow-derived stem cells.

Identification of primary structural features that define the differential actions of IL-3 and GM-CSF receptors

Activation of human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors, ectopically expressed in FDCP-mix multipotent cells, stimulates self-renewal or myeloid differentiation, respectively. These receptors are composed of unique alpha subunits that interact with common beta(c) subunits. A chimeric receptor (hGM/beta(c)), comprising the extracellular domain of the hGM-CSF receptor alpha subunit (hGM Ralpha) fused to the intracellular domain of hbeta(c), was generated to determine whether hbeta(c) activation is alone sufficient to promote differentiation. hGM-CSF activation of hGM/beta(c), expressed in the presence and absence of the hbeta(c) subunit, promoted maintenance of primitive phenotype. This indicates that the cytosolic domain of the hGM Ralpha chain is required for differentiation mediated by activation of the hGM Ralpha, beta(c) receptor complex. We have previously demonstrated that the alpha cytosolic domain confers signal specificity for IL-3 and GM-CSF receptors. Bioinformatic analysis of the IL-3 Ralpha and GM Ralpha subunits identified a tripeptide sequence, adjacent to the conserved proline-rich domain, which was potentially a key difference between them. Cross-exchange of the equivalent tripeptides between the alpha subunits altered receptor function compared to the wild-type receptors. Both the mutant and the corresponding wild-type receptors promoted survival and proliferation in the short-term but had distinct effects on developmental outcome. The mutated hGM Ralpha promoted long-term proliferation and maintenance of primitive cell morphology, whereas cytokine activation of the corresponding hIL-3 Ralpha mutant promoted myeloid differentiation. We have thus identified a region of the alpha cytosolic domain that is of critical importance for defining receptor specificity.

Modulation by growth factors of the expression of interleukin 3 and granulocyte-macrophage colony-stimulating factor receptor common chain beta c

Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific a subunit and a common beta transducing subunit (beta c). We have evaluated the effect of growth factors on the expression of beta c in normal monocytes. Addition of either GM-CSF or M-CSF to monocytes elicited a marked increase of beta c chain expression, a phenomenon seemingly related to a stimulation of the transcriptional activity of this gene mediated through an enhancement of the PU.1 DNA binding activity. Interestingly, during the activation of beta c chain expression by growth factors a switch from the synthesis of the truncated betaIT to the full-length beta c was observed. Similar observations have been made also in the growth factor-dependent erythroleukemic cell line TF-1, showing that GM-CSF deprivation elicited a marked decrease of beta c chain expression.